Electrical percentage control apparatus



May 1,6, 1950 R. s. DAvlEs A 225508,328

ELECTRICAL PERCENTAGE CONTROL APPARATUS Filed may T1, 1945 HARD STILLMANAvlEs A 1 TORNEY Patented May 16, 1950 UNITED STATES PATENT OFFICEELECTRICAL PERCENTAGE CONTROL APPARATUS Richard Stillman Davies,Philadelphia, Pa., al-

.lignor to The Budd Cm. Philadelphia, Pa.,

a corporation of Pennsylvania Application May 11, 1945, Serial No.503,171

7 Claims. ((1175-320) This invention relates to percentage indicatingand lock-out electrical circuits.

In my application, Serial No. 529,621, tiled April 5, 1944, now PatentNo. 2,472,042, of which the present application is acontinuation-inpart, I describe a circuit with associated apparatus foractuating indicating or lock-out means in a power circuit alter apredetermined percentage drop of power in that circuit. In accordancewith said application, use was made oi.' relay means directly associatedwith gas tetrodes for causing both the setting of the percentage pointand the direct energization of the control or indicating apparatus.While such an arrangement is useful in many setups, nevertheless undercertain conditions it is desirable that the speed of actuation be morerapid than is usually attainable with a mechanical relay. Also, certainimprovements in the circuits have been found possible.

Accordingly, it is an important object of the v present invention toprovide a circuit arrangement for actuating percentage indicating andlock-out means which is dependent primarily in the places where the timefactor may be critical, upon direct operation through electronic units.Another object of the invention is to provide percentage indicating andlock-out means which will function on direct application to alternatingcurrent power supply without the interposition of complete rectificationbetween the control and the power circuit. An object also is to preventcapacitor leakage during static stages of circuit operation. An objectadditionally is to provide circuitJ means for coordinating capacitordischarge cyclic intermittent flow in a related circuit. Other objectswill appear on consideration of the following description of theinvention, as applied to the form of the inventionshown in theaccompanying drawings in which: Figure l is a view of the circuit asapplied to a fully rectified current source; and

Figure 2 is a view of the modled circuit as utilized in connection withan alternating or partially rectied current source. Referring to Figure1 of the drawing, the numeral i indicates a source of alternating cur=rent which is led through the transformer 2 having a primary coil 3 anda secondary coll i to the electrodes 5 and 8 of a resistance Welder, theworkpiece between electrodes being indicated by the numeral 1. Theelectrodes 5 and 6 have connection to a transformer 8 through a primarycoil 8 the transformer having a secondarycoll I0. The terminals of thesecondary ill are connected to the anodes I i and I2 of currentrectifying tubes I8 and I4. Also the midpoint l5 of the secondary isconnected to one terminal I6 of the control circuit; and the twocathodes Il and I8 oi' the rectiiler tubes are connected through a.choke coil I9 to the other terminal 20 of the control circuit.Capacitors 2| and 22 bridge the circuit on either side of the choke coilI8, the choke coil and capacitors together forming a fil' ter to smooththe rectified current.

The welding circuit in addition to the welding transformer 2 includesalso the primary circuit control relay 23 having contacts 24 and 25 andcontact plate or switch 28 and also the initiating relay 21 havingcontacts 28, 29 and contact plate or switch 30, this latter switch beingoperated by the relay coil 3! connected to a source 32 of alternatingcurrent through the initiating switch 33 which directly or indirectly issubject to manual control as is common practice. Both of these relayswitches 2B and 30 are connected in series with 'the main power sourcethrough a timer 34, the function of the timer being to permit a fixedtiming of the full length of power application in the welding relaycircuit as determined by the operator. Since the timer is under thecontrol or the operator, it may serve any desired purpose. When thepercentage-voltage control apparatus is Aused for furnishing' anindication only, the time-controlled timer M acts as a lock-out means.When the percentage-voltage control means operates as a lock-out for thecircuit, the timer 34 is employed only as a safety device, being set fora time period normally in excess oi the percentage-voltage controllock-out time period so that should the percentage-voltage control meansfail for any reason, then lock-out would be established by the timer 34.It is pointed out that the illustration of the relay switches and timerin connection with the welding transformer 2 is merely diagrammatic,since the circuit of the welding primary is controlled by apparatususually included in the timer itself, such as means for breaking thecircuit of the control elements oi inversely connectedthyratron-controlled ignitrons usually included in the primary circuit.

Proceeding with the description o1' the percentage control means, thereare shown two gas tetrodes 40 and Il, the anodes 42 and I3 respectivelyof which are connected through conductors 44 and 60 respectively and amanual switch 45 to the positive terminal I6 of a source of directcurrent. The cathodes 41 and 48 of tubes 40 and 4i are joined togetherand to a conductor 48 connecting the cathodes to the negative terminal50 of the source 5i. The screen grid 52 of tube 46 is connected to thenegative electrode voltage terminal i6 and the screen grid 53 of tube 40is connected to a terminal 54 movable in contact with a resistor 55 ofuniform resistance per unit length bridging the main terminals I6 and20. This resistor is the percentage resistor of the circuit and it isapparent that for any given value of resistance of the resistor thepoint of contact of the slider 54 will measure percent dependent uponits positional point of contact.

The anode circuit, including conductor 44 of the tetrode 40, is thedirect current actuating circuit of the control, there being included inthis circuit a resistor 56, a potential drop occurring in the same whencurrent is flowing in the conductor 44 which is sufficient to operateamplifying means as indicated by the box 51, and thereby the relay coil58 of the relay 23. Energization of this coil causes an opening oftheswitch 28 of this relay from its normally closed position. The amplifier51 may be of any desired construction suitable for direct currentampliiication, and since these circuits are well known, no details ofthe amplifying means need be disclosed. An indicator lamp 51a may bridgethe amplifier terminals to indicate when the amplifier is operathe anodecircuit of tube 4i including the conductor 80 connected to the positiveterminal 46 of source 5I is intended to provide means, in conjunctionwith related apparatus, for fixing the percentage point for any giveninitial voltage at terminal points 6I and 62 of the percentage resistor55. To accomplish this function, the control grids 63 and 64 of tetrodes40 and 4I are connected together at point 69 and thence through resistor1o to the cathode sa of a mode vacuum tube 65. the anode 68 of the tubehaving connection to resistor terminal point 62 and also to point 61 ina conductor connecting the tetrode cathodes 41 and 48. A resistor 1i isconnected to the cathode 68 of tube 65 on the side opposite the resistor10. Two leads from a direct current source 12 connect with the ends ofthe resistors 10, 1I distant from their connection with the cathode 68,the resistors and 1i thus forming a voltage divider in the circuit fromthe direct current source 12. A resistor 13, connected at one end at apoint between the resistor 10 and point 69 and connected at the otherend to the conductor 49, is also provided.

Control of current flow through tube 65 is by means of its control grid14 which is part of a circuit including a manual switch 15 connected tothe negative terminal of source 'i2 and a capacitor 16 connected betweenthe control grid 14 and the cathode 68 of tube '55.A There is alsoprovided triode vacuum tubes 11 and 18 and the rectifier tube 19, thesethree devices being substantially in series between the capacitor 18 andthe cathode 68 of tube 65 on the one side and the conductor 60 of thecircuit of tetrode tube 4i on the other. Point 80, forming the positivecharging terminal of the capacitor 16, is connected by a lead to theanode 8i of tube 11, the cathode 82 having connection through a resistor83 to the anode 84 of tube 18 and the cathode 85 of this latter tubebeing connected through the rectifier tube 18 to the conductor 68. Thecathode 82 of tube 11 is also connected through a resistor 86 to thenegative side of the capacitor 16 at a point betweenthe switch 15 andthe grid 88 oi' tube 18 is connected to conductor 80.

14 of tube 88. The grid 81 of tube 11 is connected directly to the anode84 of tube 18 and the grid A resistor 88 is interposed in the conductor80 bv tween the control grid 88 and the cathode of rectifier 18 in theconductor 88. Also an auxiliary source of direct current 88 is connectedto the anode and cathode of tube 18, the positive terminal havingconnection to the anode through resistor 83 and the negative terminaldirectly to the cathode.

The operation of the circuit of Figure l will now be described asapplied to a conventional resistance welder. In the copendingapplication of Herbert VanSciver, Serial No. 524,243, iiled February 28,1944, now Patent No. 2,433,967. for Method of weld control and assignedto the same assignee as this application, it is pointed out that when aworkpiece, such as 1 of Figure 1, consisting of two overlapping metalplates interposed between the electrodes, is subjected to a heatingcurrent, on fusion the resistance of the workpiece drops with anaccompanying drop in voltage which may be as much as 25% of the voltageprior to fusion. Accordingly, it is apparent that if the control circuitis so devised as to utilize a predetermined percentage drop across theelectrodes for actuating indicating or lockout mechanism it would bepossible to utilize the mechanism successfully for any variation ofapplied voltage to the welding electrode terminals and to maintain thepoint of operation indefinitely at a iixed point. If the percentagecontrol be set for operation on the drop of 25% from the applied voltageat the electrodes then the lockout or indicating means would function atthis percentage irrespective of any changes of the circuit values. Toaccomplish this result, it is necessary to set up a device for selectingthe desired percentage value, and this is accomplished by the resistor55 in conjunction with the slider 54. In addition, it is necessary thatthe circuit fix the percentage point irrespective of any change in thesource voltage but dependent only upon the voltage as applied initiallyto the resistor terminals. The circuit for accomplishing iixing of thepercentage value is that including the conductor 60 and tetrode 4|.

After iixing the percentage value, it is necessary to provide anauxiliary circuit. On change of the amount of applied voltage from itsinitial value to that of the fixed percentage value, energization willoccur in a circuit which when amplified will cause signal indicating orlockout mechanism to function. This is accomplished by the circuitincluding the tetrode 48 with its associated conductor 44 andotherincluded elements.

lt is observed in the first place that the manual switches 33, 15 and 45are interconnected or ganged and simultaneouslyl operable to bring aboutthe functioning of the apparatus and circuit. Switch 15 is normallyclosed and source 12 is, prior to the initiation of a welding cycle,supplying current which passes through the voltage divider 10, 1i andthrough the capacitor 18 and switch 15 so that the capacitor 18 ischarged in the full value of the drop across resistor 1i. At the sametime, current from this source passes through the resistor 13 and tube85 to a limited amount, the current through this circuit being reducedby the negative condition of the bias on the grid 14 of the tube. Alsocurrent will be passed through the tube 18 from source 80 due to thefactthat the grid 88 has no charge.

accenna s On manipulation of the gauscd switches 33, 1l and", theclosure of initiating switch 33 operates relay 21, closing switch 30 andcausing current to ilow through the welding transformer 2 and electrodesand 3. This will cause the now of full wave rectified or substantiallydirect current transmitted from the rectifying tubes and filterarrangement to points 3| and 020i the percentage resistor; and, for thevoltage at these points, the point 54y establishes a definite percentagevoltage value. At the same time,.the

closure of gang switch 45 results in the applica-` tion of a positivepotential from source 5| on the anodes of tetrodes 40 and 4|. No currentpasses through these tubes since the flow of current is dependent uponthe proper degree of bias of both control and screen grids and it isapparent that while the `screen grid 53 of tube'4l is at the percentagevalue, the screen grid 52 of tube 40 is substantially below this value.Itis also apparent that since the control grids 63 and 04 of thetetrodes are connected together, the tube 4| will function and conductcurrent only when the bias on the control grid has been lifted to suchvalue as, in conjuntion with the fixed value o the screen grid bias, tocause the tube to sti-lire. Tube 40 will strike only when the bias onthe screen grid 52 has been changed to the same value as at the point 54initially. In other words, if the control grids of the two tetrodes aremaintained constant at a point at which tube dI strikes then on areduction of the voltage between points 8i and S2 to a point equivalentto that between points 54 and 62 before the change, tube 4i) will strikeand the actuating mechanism will be en ergized.

On the closing of switches 33 and 45 and the accompanying opening ofgang switch 15, capacitor 16, which has already been charged aspreviously described, begins to discharge through the tube 11 andresistor 86. This reduces the bias on the control grid 14 of tube 65 andincreases the now of current through this tube and thereby progressivelycharges the bias on the control grid 04 of the tetrode 4|. Thiscontinues until the tube 4| strikes and thereupon e. voltage drop isimpressed across the resistor 89 to place a positive potential on grid83 of tube 'i8 thereby inn creasing the ow of current through tube 18and the resistor 83. This increases the bias on grid 31'of tube 11, thusarresting the discharge of cur-n rent from the capacitor 16 and fixingthe poten tial on the grid 64 of tetrode 4|.

The current and voltage condition of the coni trol circuit is now staticassuming a substantially constant voltage across the percentage resistor55- and substantially no leakage from the condenser, asis the case.

Should however the voltage across resistor 55 drop for any reason suchas the completion of a weld, the voltage at point 6| with reference topoint 62 rises making screen grid 52 less negative., If the degree ofelevation is such as to bring the potential of grid 52 to the strikingpoint current will flow in the tetrode 40 producing a potential dropacross the resistor 53. This drop is in turn -ampliied by the amplifier51 and relay coil 53 is-energized opening switch 23 and thereby theprimary welding circuit including the welding primary 3 thus stoppingthe welding action. The switch 33 may then be opened thus de-energizing' the initiating relay switch 21. Switch 45 is opened and switch 15closed at the same time and, source |20 being constantly available,capacitor 'Il' is again charged.

In Fig. 2 I have shown a modification of the invention wherein points|00 and i0 I, corresponding to points i0 and 20 in Fig. l, are directlyconnected to an alternating current source which may be associated withthe secondary of a welding circuit. In this modification the alternatingcurrent atterminals |00 and |0| is preferably amplilied by transformer|02 and the transformed current led to the terminals |03 and |04 oi thepercentage resistor |05, there being connected between point |03 and thetransformer secondary, a halt-wave rectifying tube |03. The .positiveanode of the rectifying tube is connected to the point |03. Alsoconnected to the points |03 and |04 of the resistor are the gas tetrodes|01 and |08 corresponding to the tubes 40 and 4| 'of Fig. l. Tube |01includes the anode |03, screen grid H0, control grid and cathode ||2,and tetrode |00 includes the anode H3, screen grid ||4, control grid H5and the cathode H0. Connection of point |03 of the resistor is to thetube grid |83, and connection of the point |04 is to both cathodes ||2and i it of the tubes. The con trol grids of the tubes areinterconnected by the conductor i il. The screen grid |4 oi tube |08 isconnected by conductor ||3 to the movable contact H9 engaging theresistor |05 whereby variation in the potential of the grid may besecured.

is in Fig. l., there is employed a branch circuit indicated by theletter A including a source |20 of direct current adapted to passcurrent through a voltage divider including the resistors |24 and |22,with the capacitor |23, together with the manual normally closed gangswitch |24, in parallel with the resistor |22. By this means thecapacitor |23 is made to assume a. charge at a voltage equal to thevoltage drop of the resistor |22. In parallel with the resistor |2| ofthe voltage divider circuit is a second resistor |25 and a vacuum triodetube |26, the anode of the tube having connection to the resistor |25 ata point i2? and the cathode of the tube having connection to the voltagedivider intermediateresistor t2| and |22. The control grid of tube |26is connected directly to the capacitor |23 and manual switch |24. Alsopoint |21 of the resistor |25 is directly connected to point |28 in theconductor joining the tetrode cathodes and beyond that to point |04 o1the resistor |05. Also point |29 of resistor |25 is connected to theconductor ||1 joining the tetrode control grids.

The branch circuit generally indicated by B is intended to provide meansfor initiating the current now through the tetrode |03 and to x thevalue or' the current flow as established, this action being inconjunction with the capacitor |23 with its auxiliary circuit elements.The two opposite terminals |30 and |3| of capacitor |23 are connected inthe circuit including the resistor |32 and the triode vacuum tube |33,the anode of the tube being connected to the point |3| and the cathodeto the resistor |32. The cathode of the tube |33 is connected through abiasing resistor |33 to the control grid of the tube. A biasing currentis supplied for the grid of tube |33 from a source |35 of directcurrent, the biasing circuit including the source, the resistor |34, anda tube |35 having the anode thereof directly connected to a pointbetween the resistor and the grid of tube |33, and the cathode thereofto th source |35.

Also, for the tube |36 a control grid |40 is provided with a. source ofbiasing potential, in this case the means comprising a variable resistor|31 supplied by a source of direct current |38, with the slidablecontact |33 of the resistor directly connected to the control grid |48of tube |36. Further biasing control of tube |38 is provided by a secondor screen grid |4| which is connected to a biasing resistor |42 and arectifying tube |43 having the anode thereof connected directly to thecathode of tube |38 and the cathode connected to the low potential sideof resistor |42.

The resistor |42 on the low potential side is connected by conductor |44to the anode ||3 of the gas tetrode |08 and on the high potential sideto the cathode of a rectifier |45 having the anode thereof connectedthrough the secondary |48 of a transformer |41 to the negative terminalof a source of direct current |48. The positive terminal of the source|48 is also connected through the tube |49 and phase change unit |5027to the primary |52 of transformer |41 and thence to the negativeterminal source |48. The positive inside end of the transformer primary|52 at point |53 is connected also to the primary |54 of a transformer|55 having a secondary |58. This secondary |56 forms part of a circuitincluding a voltage divider resistor |51, the variable contact |58thereof being connected directly to the shield grid |59 of tube |36. Ata point |60 between the negative terminal of rectifier |45 and theresistor |42, connection is made to the anode |09 of the tetrode |01 bymeans of conductor |8|. In this circuit is included the resistor |82having terminal connections |63 and |64 through which connection is madeto the amplifier 51 and circuit breaking mechanism in the powersupplying circuit similar to the showing of Fig. 1 of the drawing.

The grid of tube |49 is directly connected by the conductor |85 to theend of the secondary of transformer |02 having connection to therectifier |08. A return conductor |68 from the oppo site side of thissecondary connects directly to the negative terminal of the source |48.The manual switch |24 in the capacitor circuit is connected foroperation with an initiating switch |81 in a starting circuit includinga power source |68 applicable through the welding circuits to theprimary circuit of transformer |02. Switch |61 of Fig. 2 corresponds toswitch 33 of Fig. 1. The branch circuit, including the rectifier |45,transformer |41, source |48, triode |49 and phase change unit |50b, isdesignated by the letter C.

The operation of this modification of the invention is as follows. Itmay be assumed first that the initiating switch |61 is open and thecircuit ||0| is de-energized and that power is being supplied thecircuit from direct current source in branch A and sources |35, |38 and|48 in branch B. As a result capacitor |23 in the A branch assumes thevoltage drop of resistor |22 from source |20 since switch |24 is closed.Also the potential on control grid |40 oftube |36 is adjusted so as tobring the grid voltage to the cut-off point thereby through effect onthe grid of tube |36 preventing current flow in tube |33 and leakagefrom capacitor |23. At the same time voltage from source 48 passescurrent through the prima-ry of transformer |55 but no transformationresults due to the direct nature of the current.

If power is now applied to terminals |00|0| through closure of switch|61 there results an irnpressment of half-cycle current pulses on thepercentage resistor |05, the corresponding voltages of the screen grids||0 and ||4 of tubes |01 and |08, resulting from this current flow,being divergent by the amount of voltage drop as determined by theposition of the resistor slide ||8.

At the same time the alternating current is impressed on the grid oftube |48 in the C branch thereby developing an intermittent currentwhich is transformed into an alternating current in transformers |41 and|55. An alternating voltage is thereby impressed upon the grid |89 oftube |38 and an intermittent positive voltage on the anode I3 of tube|08.

Since alternate half-cycles are effective at the grids ||0, ||4 of thetetrodes |01 and |06. it is desirable that the striking point in thesetubes as developed by the capacitor |23 in the A circuit occur duringthese active half-cycles to secure accuracy in the operation. This isaccomplished by the grids |59 and |4| of tube |38.

It will be apparent that since slider |38 of grid |40 is adjusted toplace the grid |40 of tube |38 at the cut-off point, on a negativealternation of the voltage in grid |59 some current will flow throughthe tube |38 and the voltage of the grid oi' tube |33 will become morepositive permitting current to flow from the capacitor |23 when theswitch |24 is opened. On the positive half-cycle of grid |59 thisdischarge ceases. Hence if the phase of voltage on grid |59 is 180displaced from that of the screen grid voltages of the tetrodes, tube|08 will strike only during discharge from the capacitor |23.

If the initiating switch |81 is now closed and the gang switch |24 isopened, capacitor |23 in the A branch begins to discharge cyclicallythrough tube |33. As the voltage across the capacitor decreases the gridof tube |28 becomes more positive increasing the current flow throughresistor |25 and raising the bias of the control grids of tubes |01 and|08 until tube |08 strikes. Thereupon current flows through this tube|03 bringing a voltage drop across resistor |42 in conductor |44 andapplying an intermittent voltage to grid |4| 180 out of phase with thevoltage on grid |59, neutralizing the same. Thereupon the bias of thegrid in tube |33 falls to the cut-ofi' point thereby stopping the flowof current from the capacitor |23. A static condition then exists in thecircuit of tetrode tube |08, assuming an average constant voltage acrossboth terminals |00|0|.

In event that the voltage across |00|0| drops as in the case of weldcompletion in a welding circuit, the bias on the screen grid 0 of tube|01 is reduced; and, if this bias reduction is sufficient, tube |01strikes and a. voltage drop appears across resistor |62 which ifamplified by the connected circuit as shown in Fig. 1 may be utilizedfor indicating the voltage drop in the supply circuit or forinterrupting this circuit as by use of a disconnecting relay in thesupply circuit. The particular utility of the modification as shown inFig. 2 is to reduce the time factor by elimination of part of thetransformation apparatus.

Modifications other than hereinabove described may be 'made andinterchanges and substitutions carried out which form known usage amongengineers and technicians in this field and hence no limitation isimplied by the specific circuits illustrated and described other thanthat which may be required by the claims hereto appended.

What is claimed is:

1. In an electric control system for an alternating current electricalpower unit subject to voltage change, circuit means adapted forconnection to the power unit for establishing a fixed referencepercentage voltage value of the power unit 7,5 voltage. said circuitmeans comprising a rectier.

asoaaas a resistor connected to said power unit through said rectifier,a movable element engaging said resistor for functioning as a voltagedivider, a capacitor, a circuit connected between said resistor andcapacitor subject to intermittent voltage pulses from the power unitthrough the rectifier, means for charging said capacitor, means forfixing the charge of the capacitor at a predetermined value dependent onthe initial voltage of the power unit, said charge-fixing meansincluding means for discharging said capacitor only during said voltagepulses on said circuit, and control means jointly responsive to thesubsequent changed voltage of the power unit and a potential establishedthereon as a standard reference value from the initial power unitvoltage for providing a control impulse.

2. In an electric control system for an alternating'current electricalpower unit subject to voltage change, a conductor connected to saidpower unit, circuit means adapted for connection to the power unit forestablishing a fixed reference per= centage voltage value of the powerunit voltage, and control means connected to said circuit means andoperative at a change oi the unit volt age with reference to said fixedpercentage volt= age to alter the encrgization of said conductor, saidcircuit means comprising a rectifier, a resis-s tor connected to saidpower unit through said rec=l tifler, a movable element engaging saidresistor ior functioning as a voltage divider, a capacitor, a circuitconnected between said resistor and said capacitor subject tointermittent voltage pulses i'rom the power unit through the rectier,means for charging said capacitor. means for fixing the charge of thecapcitor at a predetermined value dependent on the initial voltage ofthe power unit. said charge-xing means including means for dischargingsaid capacitor only during said voltage pulses on said circuit, andmeans jointly respon@ sive to the subsequent changed voltage of thepower unit and a. potential established thereon as a standard referencevalue from the initial power unit voltage for providing a control im=pulse.

3. In an electric control system for an electrical power unit subject tovoltage change during a given operation, a control circuit connected tosaid power unit, electronic comparator means connected in said controlcircuit, said comparator means having a rst part responsive to apercentm age of full voltage and a second part responsive to iullvoltage, means including an initially charged capacitor and dischargemeans therefor for causing current to iiow in said first electroniccomparator part when the capacitor is discharged to a pointcorresponding to the percentage voltage level prior to change, meansincluding an elece tronic tube which is controlled by current iiow insaid iirst comparator part for halting the dis1 charge of said capacitorand thereby fixing its charge at a given level corresponding to saidpercentage voltage level, and voltage biasing means between saidcapacitor and said second comparator part for causing current to flow insaid second comparator part when the full volt= age thereon reaches apredetermined relationship with respect to the voltage exerted thereonfrom said capacitor.

4. In an electric control system for an electrical power unit subject tovoltage change during a given operation, a control circuit connected tosaid power unit, a voltage drop resistor connected across said circuitand an adjustable percentage voltage drop or voltage divider deviceconnected with said resistor, electronic comparator means in saidcircuit including a first comparator part responsive to percentagevoltage drop as controlled by said adjustable device and a secondcomparator part responsive to i'ull voltage drop, a capacitor in saidcircuit having biasing connections to both of said comparator parts,means to give said capacitor an initial charge sumcient to prevent flowof current in said first comparator part, means including an electronictube which passes current in accordance with the charge on saidcapacitor and which exerts a hold-oiI-opposing bias on said comparatorparts, means for cutting off said capacitor from its charging means atthe start of an operation, means including an electronic tube fordischarging said capacitor at the beginning of an operation and therebyincreasing the hold-off-opposing bias until current iiows in said firstcomparator part. means including an electronic tube which is actuated bycurrent flow in said first comparator part for halting the discharge ofsaid capacitor and thereby fixing its charge at a given level, saidvoltage hold-ofi-cpposing biasing means between said capacitor and saidsecond comparator part causing current to low in said second comparatorpart when the full voltage hold-ofi' bias thereon drops to that exertedby the capacitor with its fixed charge.

5. A controi system es set forth in claim. 3 which further includesmeans responsive to current ow in said second comparator part forcontrolling the operation ci said power unit.

6. In an electric control system for an electri cal power unit subjectto voltage change, circuit means adapted for connection to the powerunit for selectively establishing a fixed reference percentage voltagevalue of the power unit voltage, said circuit means comprising a linearresistor connected to said power unit, a movable element engaging saidresistor for functioning as a voltage divider, a capacitor, anelectronic device connected between and subject to the conjoint opposingbias of said movable element and capacitor and subject to current flowtherethrough when the bias voltage produced by said capacitor equalsthat produced from said movable element, an electric power source forsaid device, means for charging said capacitor, means for dischargingsaid capacitor to a point required to initiate current iiow in saiddevice, and means including an electronic tube for arresting thedischarge of said capacitor when a current flows in said device, wherebycurrent flow through the device is made to continue and the charge onsaid capacitor is kept constant, and control means, including a secondelectronic device jointly responsive to voltage bias from said capacitorresulting from iiow of current in said first device and to full voltageat the power unit for providing a control impulse upon a change of fullvoltage to a given value relative to the percentage voltage.

7, In an electric control system ior welding equipment or similarelectric power units subject to voltage change in use, circuit meansadapted for connection to the power unit for establishing a full voltageand a selectable fixed reference percentage voltage value of the powerunit voltage, said circuit means comprising a iinear resistor connectedto said power unit, a movable element engaging said resistor forfunctioning as a voltage divider, a capacitor, a, first electronicdevice, having grids connected respectively to said 1i movable elementand to means to the charge on said capacitor, m'electric power sourcefor said first electronic device. moans for charging said capacitor,means including n first thermionic tube having a Urid for seid capacitorto'initiete current now in' said rst electronic device, means includingn second thermionic tube responsive in action to current now in saidiirst electronic device for arresting the discharge of said capacitorartnr current flows through the iirst electronic device lor fixing at aconstant value the charge on said capacitor, a second electronic devicehaving a grid connected to means responsive to the charge on saidcapacitor for establishing -a iixed bias on said second electronicdevice as a percentage of the initial voltage from the power unit, und asecond grid tor said second electronic device connected to said powerunit across the full resistor for biasing the second electronic deviceto produce flow of current in said second electronic device when thevoltage from the power unit changes to s l2predeierminedvsldiilerentfromthoiitial valueandtoseidperoentsgsvolt- Thefollowing references are of record in the tile o! this patent:

UNITED STATES PATENTS Number Nemo Date 1,854,965 Trsver' Apr. 19, i9391,931,069 Fitzgerald Oct. 17. 1933 1,959,690 Roth May 22, 1934 1,967,8504 Wideroe July 24, 1934 2,169,851 Romnes Aug. 15, 1939 2,306,593 CollomDec. 29, 1942 2,400,259 Place May 14. 1946 FOREIGN PATENTS NumberCountry Date 113,839 Australia Sept. 10, 1941

